Variation in an intertidal subcanopy assemblage dominated by the rockweed Pelvetia compressa (Phaeophyceae, Fucales)

Variation in population and community parameters was examined for the Pelvetia compressa (J.Ag.) DeToni subcanopy assemblage at eight sites, distributed over ca. 15.5-km of relatively uniform southern California coastline. Seaweed and sessile invertebrate cover and mobile invertebrate densities were determined in plots randomly located on horizontal surfaces during fall 1995 and spring 1996. A total of 47 seaweeds and 63 macroinvertebrates were encountered during the study; however, few species consistently dominated the subcanopy assemblage. Although oceanographic conditions were similar, maximum wave force, sand influence, and substratum microrelief varied significantly among sites. Patterns of variation among sites were difficult to detect at the community level based on detrended reciprocal averaging, and site patterns were only weakly associated with either biotic or abiotic variables. However, significant differences among sites were detected in the most abundant populations. Two crustose algal species contributed the majority of subcanopy cover at all sites, which were also characterized by small shifts towards either greater abundances of sessile invertebrates, limpets and chitons or articulated coralline and fleshy algae. Four sites have received protection as marine reserves for more than 25 years, but no relationship was found between reserve status and species abundance patterns. These results underscore the difficulties in establishing relationships between key environmental parameters and community patterns over small horizontal scales, and the importance of local and variable disturbance histories in generating variation among sites.